Passive infrared motion detector are well known in the art. Such detectors typically include a small housing, a lens for directing infrared light from a zone to be monitored onto an infrared sensor element. The most common type of sensor element is a pyroelectric sensor which generates a small but detectable electrical voltage in response to changes in infrared radiation impinging on it. Due to the lens of the known detectors, motion of a person into or from the monitored zone changes the intensity of infrared light striking the sensor and produces a voltage variation. A discriminator circuit monitors the sensor output signal and generates an alarm signal provided that certain characteristics are present in the sensor output signal. Such characteristics may be energy, amplitude peaks, number of oscillations, etc.
An example of the prior art detectors is found in U.S. Pat. No. 5,077,549, co-invented by the inventor of the present invention. It is known from this reference to provide an infrared motion detector circuit which uses a digital logic circuit to discriminate the sensor output signal. The sensor signal amplitude is converted by a voltage controlled oscillator (VCO) to a pulse frequency, and a counter circuit counts pulses to detect the sensor signal's energy and generate the alarm signal.
In the past, the use of a microprocessor along with an analog-to-digital converter (ADC) has been prohibitively expensive for use in a commercially competitive motion detector discriminator circuit. In today's market, various commercially available microcontrollers or microprocessors are inexpensive enough to competitive with standard "hard wired" integrated circuit technology. While the use of a microprocessor improves the flexibility and possible quality of the signal analysis and discrimination, it still requires circuitry for generating a digital signal from the sensor's analog output, such as a high gain bandpass amplifier and an ADC, which represents a significant part of the present cost of the detector circuitry.